The present invention relates to improving the yield of a polycrystalline silicon production system by increasing the useable length of polycrystalline silicon rods produced.
Polycrystalline silicon is a critical raw material for the electronics industry. It is the starting material for producing single crystal silicon ingots for the semiconductor industry. These ingots are produced either by the Czochralski (CZ) or Float Zone (FZ) method.
In the CZ crystal pulling process, chunks of polycrystalline silicon are loaded into a quartz crucible. The chucks of polycrystalline silicon are random size varying from 14 inches in length. In order to maximize the packaging density of the polycrystalline silicon, polycrystalline chips, granules or short rod pieces may also be added to the quartz crucible to increase the packing density. The crucible is filled, loaded into the CZ furnace and the polycrystalline silicon is melted.
Upon melting, due to interstices among the polycrystalline silicon pieces packed in the crucible, 20-30% of the crucible volume is unfilled. This can have a significant impact on the overall yield of the CZ process. In order to maximize the yield of single crystal silicon from the melt, there is a variety of options to top off the crucible and thus increase the silicon melt volume. Small chunks, chips or granular polycrystalline silicon can be added via a quartz tube. These sources have a much higher surface area than the initial polycrystalline silicon chunk and therefore have a higher potential to add surface contaminates to the melt. In addition, many CZ single crystal silicon growers have difficulty obtaining good yield if they add more than 10-20% of the total weight of the charge as granular polycrystalline silicon.
An alternative method has been to use polycrystalline rods to top off the crucible. This process is commonly referred to as charge replenishment (CR). CR rods are typically  less than 900 mm in length and tip to 35-40 kg in weight.
In the FZ method, rods of polycrystalline silicon are converted into single crystal by float zoning. The FZ process is a crucible-free process in which a polycrystalline silicon rod is melted using a RF field, which acts as the energy source, as well as the containment field. This results in the ability to melt the polysilicon and grow single crystal silicon without the use of a crucible. Typically, in the FZ process, polycrystalline rods of 1500-2000 mm in length and 75-150 mm in diameter are loaded into the FZ puller and single crystal ingots of 75-150 mm in diameter and  greater than 1500 mm in length are produced.
In the FZ process a critical parameter for yield, particularly for 125 mm or greater diameter ingots, is the availability of large diameter, full-length polycrystalline silicon rods. These polycrystalline silicon rods need to be similar in diameter and length to the ingots to be pulled in order to maximize yield. Most of the major FZ ingot growers who produce greater than 125 mm diameter ingots use equipment designed to pull  greater than 2000 mm length ingots, therefore, the polycrystalline silicon rods must be similar in length or greater depending on diameter to optimize yield of single crystal.
Polycrystalline silicon rods for both the FZ and CR applications must be free of surface cracks and spalls. Cracks can cause the rods to break during processing due to thermal as well as internal stresses. Such breakage can cause extensive damage and down time to the crystal growing equipment. Spalls, depending on size and location, are also detrimental to the process. This is due to the potential for cracks to be associated with the spalling. In addition, the loss of mass associated with spalls can impact the quantity of single crystal produced. In large diameter polysilicon rod production, spalling is the major failure mechanism.
In FZ applications, the geometric relationship between the polysilicon rod and RF coil needs to be very consistent in order to achieve acceptable product quality and yields. Due to the critical nature of this relationship, spalls on virgin polysilicon rods must be completely removed in order to be acceptable for use in the FZ process.
In CR applications some spalling can be tolerated. However, due to physical constraints within the CZ pulling equipment it is desirable to have consistent rod length-to-mass ratios. Rod spalls lead to variability in the rod length/mass ratios that must be compensated for during processing, which can lead to reduced productivity.
The production of polycrystalline silicon rods by the pyrolytic decomposition of a gaseous silicon compound, such as silane or a chlorosilane, on a suitable substrate is a well-known process. The process typically comprises:
a) An even number of electrodes are attached to a base plate, each electrode can have a starting filament (starter rod) attached. The filament is on the order of 2000 mm in length.
b) The filaments are joined in pairs by connecting bridges. Each bridge is a piece of starter rod material and is joined to two starting filaments. Each set of two filaments and a bridge thus is a generally an inverted U-shaped member, commonly referred to as a hairpin. For each hairpin assembly, an electrical pathway is formed between a pair of electrodes within the reactor. An electrical potential applied to the electrodes can thus heat the attached hairpin resistively.
c) The hairpins arc contained in a bell jar enclosure that mates with the base plate to define a batch reactor allowing operation under vacuum or positive pressure conditions.
d) A gaseous silicon precursor compound of the desired semiconductor material and other gases, as necessary, are fed into the reactor.
e) The U-shaped members are electrically heated to a temperature sufficient to effect decomposition of the gaseous precursor compound and simultaneous deposition of the semiconductor material onto the hairpins, thereby producing U-shaped polysilicon rods of substantial diameter.
f) Any by-product gases and unreacted precursor compounds are exhausted from the reactor.
The principles of design of present state of the art reactors for the pyrolysis of silane and chlorosilanes are set forth in, for example; U.S. Pat. Nos. 4,150,168; 4,179,530; 4,724,160; and 4,826,668, each of which is incorporated herein by reference. The length of the polycrystalline rod that can be grown in a reactor is limited by the geometry of the reactor, height of the enclosure, length of the filament and the reactor power supply.
The temperature of this process is carefully controlled in order to reduce the amount of stress in the rods as they grow. Unfortunately, at each filament-bridge junction region there is generated a large amount of radiant energy that is exchanged between the filament and the bridge. This results in a large amount of induced stress in the rod. The amount of stress increases significantly as the rods become larger in diameter.
This stress can become great enough to cause the rods to fracture in this region upon cooling. This fracture is typically a spall that can be 100-500 mm in length. This reduces the available length of useable rod by almost the same length as the spall. Thus in many cases, the rod can no longer be processed for the FZ process and may not be useable for the CR rod process. If CR rod length is  less than 900 mm then the optimum yields for the polycrystalline silicon manufacturer is to produce two CR rods per polycrystalline rod. Spalling of the rods in the reactor can greatly impact this yield.
Thus there is a need to reduce the impact of spalling and to improve the yield of the silicon deposition process. This is accomplished by a process that increases useable rod length, eliminates the need to saw the rod to desired length, and/or eliminates the need to etch the rod pieces to remove the contamination from sawing.